Fix wxWindowModalDialogEvent::GetReturnCode() on Windows.

*Lots* of native dialogs don't call SetReturnCode() in their ShowModal()
implementations in wxMSW. Rather than fixing them all - and having them
accidentally broken again and again in the future - fix this just in the
generic ShowWindowModal() implementation. This may not be the most elegant fix,
but it is bullet-proof.

Also revert previous attempt to fix just wxMessageDialog in r74776.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75089 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775